Synthesis of 9-(substituted phenoxycarbonyl)-10-methylacridinium trifluoromethanesulfonates: Effects of the leaving group on chemiluminescent properties.

Various 9-(substituted phenoxycarbonyl)-10-methylacridinium trifluoromethanesulfonates possessing electron-withdrawing substituents have been synthesized. The effect of substituents on the stability of the acridinium esters (AEs) at various temperatures in different buffers and the chemiluminescent properties have been examined. There was little correlation between the chemiluminescent properties of AEs and the pKa values of their associated phenols, but the steric effects of the ortho-substituents in the phenoxy group, as well as their electron-withdrawing natures, seem to play an important role in determining the properties. In general, when two identical substituents are present in the 2- and 6-positions, the compound is significantly more stable than when only a single substituent is present, presumably because of greater steric hindrance from the second group. The exception is the 2,6-difluorophenyl ester, which is less stable than the 2-fluorophenyl ester, presumably because the fluoro group is small. Addition of a third electron-withdrawing substituent at the 4-position, where it has no steric influence, typically increases susceptibility to decomposition. The presence of a nitro group has a significant destabilizing effect on AEs. Of the AEs studied, the 4-chlorophenyl ester showed the greatest chemiluminescent yield, while the 2-iodo-6-(trifluoromethyl)phenyl ester group showed the greatest stability in low pH buffers.